1. Field of the Invention
The invention relates to a tracking-type laser interferometer.
2. Description of the Related Art
A tracking-type laser interferometer is used as an apparatus for measuring a distance to a movable body. An example of a tracking-type laser interferometer of related art is disclosed in Japanese Unexamined Patent Application Publication No. 2008-128899. The tracking-type laser interferometer disclosed in the above patent document splits a beam of laser light into a beam of measurement light and a beam of reference light. The interferometer emits the measurement light toward a retro reflector that is attached to the movable body. The interferometer performs control processing such that the shift amount of return light propagating back from the retro reflector should fall within a predetermined range, thereby keeping track of the retro reflector. The reference light is reflected at the reference plane. The reference light reflected at the reference plane and the return light propagating back from the retro reflector turn into interference light. The interferometer utilizes the interference light to measure a distance therefrom to the retro reflector (movable body).
Backward light that is supposed to propagate back from a retro reflector will not actually return to an interferometer when measurement light is not properly directed at the retro reflector or when there is an obstacle between the interferometer and the retro reflector. In such cases, the tracking-type laser interferometer of related art loses sight of the retro reflector and thus continues emitting measurement light blindly in the lost direction. For this reason, the tracking-type laser interferometer of related art cannot resume the measurement of a moveable object when the interferometer loses sight of the retro reflector, which is a problem that remains to be solved.